The present invention relates to the treatment of streams having mercaptan or mercaptan-based compounds therein; more particularly, it relates to improved methods for treating such streams by utilizing nitrogen-based promoters as additives to promote the extraction and/or catalytic oxidation of the mercaptan or mercaptan-based compounds therein.
This invention is particularly adapted to a variety of processes for sweetening mercaptan-containing sour hydrocarbon distillate, and is further adapted to the processes for regenerating spent caustic solutions utilized in extracting mercaptan compounds from such sour hydrocarbon distillate.
The art relating to the treatment of mercaptan-containing sour hydrocarbon distillate and the regeneration of spent caustic solutions is well developed and the processes and apparatus therefor are the subject of many patents. For example, U.S. Pat. Nos. 2,988,500; 3,371,031; 3,413,215; 3,445,380; 3,515,677; 3,574,093; 3,923,645; 3,931,054; 3,972,829; 4,003,827; 4,009,120; 4,018,705; 4,033,860; 4,070,271; 4,087,378; 4,090,954; 4,098,681; 4,107,078; 4,113,604; 4,121,998; 4,121,999; 4,124,531; 4,141,819; 4,206,043; 4,248,694; 4,298,502; 4,364,843; 4,481,106; 4,481,107; 4,490,246; 4,498,977; 4,498,978; and 4,579,121 are representative of catalytic oxidation processes and catalysts for treating mercaptan-containing sour hydrocarbon distillate. U.S. Pat. Nos. 2,425,414; 2,606,099; 2,740,749; 2,853,432; 2,921,021; 2,937,986; 3,107,213; 4,040,947; 4,081,354; 4,104,155; 4,199,440; and 4,362,614 are representative of extraction and regeneration processes. U.S. Pat. Nos. 2,176,806; 2,215,359; 2,299,426; 2,662,001; 3,226,092; 3,238,124; 3,351,434; 3,496,996; 3,585,005; 3,758,404; 3,839,487; 3,977,829; 3,989,466; 3,992,156; 4,019,869; 4,039,389; 4,201,626; 4,219,420; 4,364,821; and 4,491,565 are representative of apparatus useful in the practice of various of the aforementioned processes. All of the above-listed patents are hereby incorporated by reference herein for all purposes as if fully set forth.
In general, the sweetening of mercaptan-containing hydrocarbon distillate entails oxidizing the noxious mercaptan compounds to less objectionable disulfides. Commonly used treating processes provide for contacting the distillates with an oxidizing agent, usually air, and a mercaptan oxidation catalyst dispersed in an aqueous caustic solution, usually an alkali metal hydroxide solution. The sour distillate and the catalyst containing aqueous caustic solution provide a liquid-liquid system wherein mercaptans are converted to disulfides in the presence of an oxidizing agent, usually an oxygen containing gas dissolved in the hydrocarbon being treated.
Sour hydrocarbon distillate containing more difficultly oxidizable mercaptans may also be effectively treated in contact with a mercaptan oxidation catalyst deposited on a high surface area adsorptive support, usually a metal phthalocyanine on an activated charcoal. The distillate is treated in contact with the supported catalyst at oxidation conditions in the presence of an aqueous caustic solution. The oxidizing agent is most often air admixed with the distillate to be treated, and the caustic solution is most often aqueous alkali metal hydroxide. The caustic solution is charged continuously to the process or intermittently as required to maintain the catalyst in a caustic-wetted state.
Mercaptan-containing hydrocarbon distillate may also be sweetened by contacting the distillate with an aqueous caustic stream whereby the mercaptans are extracted into the caustic in the form of mercaptides. The mercaptide-containing caustic solution is then separated from the sweetened distillate and may be recycled until the caustic solution becomes spent, that is, until the caustic solution loses its capacity to extract the mercaptan compounds to such an extent that the process can no longer run efficiently.
Conventionally this spent caustic solution is either discarded or regenerated for reuse. In general, the regeneration of spent caustic solutions entails oxidizing the mercaptides to disulfides and extracting or otherwise separating out the resulting disulfides from the caustic solution. Commonly used regeneration processes provide for adding a mercaptan oxidation catalyst to the spent caustic solution then contacting the resulting solution with an oxidizing agent. The mercaptan oxidation catalyst is usually a metal phthalocyanine and the oxidizing agent is usually an oxygen-containing gas such as air. The catalyst containing caustic stream and the oxygen-containing gas provide a gas-liquid system wherein mercaptides are converted to disulfides in the presence of the gas, with the resulting disulfides being removed by the settling out thereof. The regenerated caustic stream is then separated from the settled residue for reuse.
The prior art, to a certain extent, recognizes that some nitrogen compounds may be useful in the sweetening and regeneration processes. For example, U.S. Pat. No. 2,508,817 recognizes the use of N-alkyl derivatives of paraphenylene diamine as catalysts in the oxidation of mercaptans to disulfides. More specifically, the patent teaches a batch process in which a light hydrocarbon oil suitable for motor fuels and having 0.002% or less mercaptan sulfur is contacted with oxygen dissolved in the hydrocarbon oil and from 0.002% to 0.06% by weight of the N-alkyl derivative of paraphenylene diamine also dissolved in the hydrocarbon, to convert the mercaptans to disulfides to sweeten the sour hydrocarbon oil.
U.S. Pat. No. 2,565,349 discloses the use of relatively large amounts of pyridine as a catalyst in the sweetening of sour petroleum distillate. The patent teaches that contacting the sour distillate with a mild oxidizing agent in the presence of 0.1% to 5.0% by volume pyridine and 0.1% to 5.0% by volume alkali solution at temperatures of from 60.degree. F. to 200.degree. F. will result in the conversion of some mercaptans to disulfides, thereby sweetening the sour distillate.
U.S. Pat. Nos. 3,408,287 and 3,409,543 disclose a sweetening process whereby a sour hydrocarbon stream is contacted with an oxidizing agent and a phthalocyanine catalyst in the presence of an alkali solution containing from 1.0% to 90% by volume of a polar-organic solvent selected from the group consisting of dialkyl sulfoxides, amino alcohols, amino-hydroxyalkyl ethers, alkyl amines, alkyl polyamides, alkyl amides and mixtures thereof. U.S. Pat. No. 3,409,543 further discloses regenerating the polar organic solvent containing alkali solution by conventional means.
U.S. Pat. No. 3,785,964 discloses a process for sweetening sour hydrocarbons whereby a sour hydrocarbon stream is contacted with a calcined copper-iron fixed bed catalyst in the presence of 0.0001% to 5% by weight of certain nitrogen compounds, preferably ammonia or pyridine.
U.S. Pat. No. 3,853,746 discloses the use of sulfuramides activated by a carbonyl or sulfonyl group adjacent to the sulfuramide nitrogen in the process of sweetening sour hydrocarbon distillates.
U.S. Pat. No. 4,039,586 discloses a process for oxidizing organic thiols to disulfides whereby the organic thiol is reacted with a xanthide in the presence of a tertiary amine.
U.S. Pat. Nos. 4,048,097; 4,078,992 and 4,088,569 disclose the use of an ammonium donor in the preparation of metal phthalocyanine catalyst composites.
U.S. Pat. Nos. 4,100,057; 4,142,964 and 4,168,245 disclose the use of small amounts of morpholine in the catalytic sweetening of sour petroleum distillate.
U.S. Pat. Nos. 4,121,997; 4,124,493; 4,124,494; 4,127,474; 4,156,641; 4,157,312; 4,159,964; 4,203,827; 4,206,079; 4,213,877; 4,250,022; 4,260,479; 4,276,194; 4,290,913 4,290,916; 4,290,917; 4,293,442; 4,295,993; 4,298,463; 4,299,729 and 4,308,169 disclose the use of ionic, quaternary ammonium compounds in processes for oxidizing mercaptan compounds.
U.S. Pat. No. 4,207,173 discloses the use of a tetraalkyl guanidine to supply the basic medium instead of an aqueous sodium hydroxide solution customarily used in the processes for sweetening sour hydrocarbons.
U.S. Pat. No. 4,502,949 discloses a process for sweetening sour hydrocarbons whereby the mercaptans contained in the hydrocarbon are reacted with an oxidizing agent by contacting the hydrocarbon and oxidizing agent with a supported metal chelate mercaptan oxidation catalyst and anhydrous ammonia in the absence of an aqueous phase.
U.S. Pat. No. 4,514,286 discloses a process for reducing the mercaptan concentration of a sour petroleum distillate by contacting the distillate with a hydroperoxide compound and a quaternary ammonium hydroxide salt.
The above-listed patents are also incorporated by reference herein for all purposes as if fully set forth.
None of the above-described processes or nitrogen compounds as utilized in those processes is applicable for use in a variety of processes or as both a promoter for extraction and catalytic oxidation. For example, the use of high concentrations of nitrogen compounds in the hydrocarbon stream as taught by some of the references may result in unwanted color problems with the product. Other of the processes are not applicable in conventional hydrocarbon caustic oxidizing agent oxidation catalyst systems which dominate the sweetening and regeneration operations. Further, many of the processes are not easily adaptable to such conventional systems.
It has now been surprisingly discovered that the extraction and catalytic oxidation of the mercaptan compounds is promoted by the addition of small amounts of selected nitrogen-based compounds, as described hereinafter. It has also been surprisingly discovered that such nitrogen-based compounds may be utilized in a variety of sweetening and regeneration processes, as is also described hereinafter.